An optical lens is typically made of plastic or glass material and generally has two opposing surfaces which co-operate with one another to converge or diverge light according to a required corrective prescription. When the positioning or shape of one of these surfaces with respect to the other is inaccurate, optical errors can be created.
Manufacturing of an optical lens to the required prescription requirements typically includes machining the surface of a semi finished lens or lens blank. Typically, a semi-finished lens has a finished front surface and an unfinished back surface. By machining the back surface of the lens to remove material, the required shape and positioning of the back surface with respect to the front surface for the desired corrective prescription can be generated. Further surfacing operations of an optical lens can include chamfering, bevelling, polishing, or coating the surface of a lens member in order to modify the optical surface. Cribbing of the optical lens involves providing the peripheral edge of the optical lens with the desired contour for the finished optical or ophthalmic lens.
During manufacturing of the lens it is important that the semi finished lens is securely maintained at a correct positioning on a support block during the various manufacturing operations in order to prevent the generation of optical errors.
Current techniques for ensuring that the positioning of a lens member with respect to a blocking support is as accurate as possible during manufacturing of an optical lens includes verifying the positioning during the blocking step by methods ranging from an operator visualising the lens positioning directly or by means of a camera, and then correcting the positioning if necessary to fully automated verification and positioning correcting processes. Fully automated positioning methods are becoming progressively complex and time consuming. EP 1194267 describes a method implementing machine vision and probing equipment to generate an accurate virtual entity of a lens blank. The virtual entity is used throughout the lens generation process for calculating and selecting lens angles, tool angles and tool paths that produce the best lens quality